MAKING
1. In a pan, add the watercress and stock. Simmer for a few minutes.
2. Let it cool down a bit, then work to a smooth puree in an electric blender. Let it cool down to room temperature.
3. Mix in the gelatine, then whisk in the cream.
4. Season with salt and pepper to taste.
5. TO STORE BY FREEZING: Pour into a rigid container. Seal the container, label it and freeze.

SERVIING
6. TO USE A FROZEN SAUCE: Thaw at room temperature for 4 hours, then beat until smooth. Serve with roasted vegetables.